This invention pertains to chemical-mechanical polishing and a method of monitoring the polishing process.
Chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d) processes are used in the manufacturing of microelectronic devices to form flat surfaces on semiconductor wafers, field emission displays, and many other microelectronic substrates. For example, the manufacture of semiconductor devices generally involves the formation of various process layers, selective removal or patterning of portions of those layers, and deposition of yet additional process layers above the surface of a semiconducting substrate to form a semiconductor wafer. The process layers can include, by way of example, insulation layers, gate oxide layers, conductive layers, and layers of metal or glass, etc. It is generally desirable in certain steps of the wafer process that the uppermost surface of the process layers be planar, i.e., flat, for the deposition of subsequent layers. CMP is used to planarize process layers wherein a deposited material, such as a conductive or insulative material, is polished to planarize the wafer for subsequent process steps.
In a typical CMP process, a wafer is mounted upside down on a carrier in a CMP tool. A force pushes the carrier and the wafer downward toward a polishing pad. The carrier and the wafer are rotated above the rotating polishing pad on the CMP tool""s polishing table. A polishing composition (also referred to as a polishing slurry) generally is introduced between the rotating wafer and the rotating polishing pad during the polishing process. The polishing composition typically contains a chemical that interacts with or dissolves portions of the uppermost wafer layer(s) and an abrasive material that physically removes portions of the layer(s). The wafer and the polishing pad can be rotated in the same direction or in opposite directions, whichever is desirable for the particular polishing process being carried out. The carrier also can oscillate across the polishing pad on the polishing table.
The CMP process desirably results in a planar surface on a wafer with little or no detectable scratches or excess material present on the wafer surface. Precise control of wafer planarization is required during the CMP process, and it is therefore necessary to periodically, if not continuously, monitor the wafer in order to ensure sufficient but not excessive polishing of the wafer. The point at which excess material on a wafer surface is removed, but desired material remains, is called the xe2x80x9cendpointxe2x80x9d of the CMP process. Overpolishing (i.e., removing too much) of a wafer can damage the wafer surface, rendering the wafer unusable. Underpolishing (i.e., removing too little) of the wafer requires that the CMP process be repeated, which is inefficient and costly. Moreover, underpolishing sometimes is not noticed, which can cause subsequent processing difficulties and eventually render the wafer unusable. The time interval between a state of underpolishing and overpolishing can be small, e.g., on the order of a few seconds. Thus, accurate in situ endpoint detection is highly desirable.
While the polishing endpoint for a substrate can be estimated using the polishing endpoint for a previous substrate of the same type, the estimated endpoint may not be accurate because polishing conditions can change. Similarly, removing the substrate from the polishing pad and substrate carrier and measuring the change in thickness of the substrate in an effort to determine the polishing endpoint is time-consuming and can damage the substrate, thus reducing the throughput of the CMP process.
Standard techniques currently used for in situ endpoint detection include optical reflection, thermal detection, and friction-based techniques. Optical reflection techniques encounter higher levels of signal noise as the number of process layers increases, thereby decreasing the accuracy of endpoint detection to a point where the endpoint cannot be detected. Optical reflection techniques may also require that the wafer be moved off the edge of the polishing table, thus interrupting the polishing process. In addition, this can cause the endpoint to be missed and its detection delayed by perhaps as much as a few seconds, depending on oscillation speed and distance.
Thermal imaging involves the remote sensing of temperature across the wafer using techniques such as pyrometry, fluoroptic thermometry, and laser interferometric thermometry. Thermal techniques suffer from thermal noise caused by variations in the wafer production rate, variations in the polishing composition, or changes in the polishing pad. Thermal techniques are also adversely impacted by complexity in the thermal variations as the CMP tool warms and cools over the operation cycle and carrier arm oscillations.
Friction-based techniques detect the endpoint by monitoring the change in the friction coefficient between the substrate surface and the polishing pad. For example, the coefficient of friction is different for a conductive metal sliding on the polishing pad versus an insulating oxide sliding on the polishing pad. The level of friction can be measured by several methods including monitoring the friction force, monitoring the power consumed by the CMP tool""s carrier or platen motor, or by measuring the changes in torque of the carrier shaft. Friction-based techniques are satisfactory when there is a significant change in friction as the underlying layer is exposed; however, they also have their drawbacks. For many applications, the change in friction associated with the interface between layers is too small to result in a change sufficient to be a reliable indicator of the CMP process endpoint. This is particularly a problem when there is little difference between the materials of two layers. For example, a small pattern factor (that is, a relatively small area of the underlying patterned layer, compared with the area of the entire layer) causes only a small change in friction as the endpoint is reached, thereby limiting the useful signal. The problem can be further compounded by a large noise component. Indeed, even with filtering, the power signals may have complex shapes that mask the relatively simple change caused when the endpoint is reached.
Accordingly, there remains a need for an improved method of monitoring the polishing process. The invention provides a method and apparatus that address at least some of the aforementioned problems. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The invention provides a method for monitoring a chemical-mechanical polishing process comprising receiving a real-time data signal from a chemical-mechanical polishing process, wherein the real-time data signal pertains to a frictional force, torque, or motor current, submitting the data signal to an algorithm that separates the data signal into a power spectrum of signals with different frequencies, identifying at least one signal component of the power spectrum corresponding to an aspect of the chemical-mechanical polishing process, monitoring the signal component of the power spectrum during the chemical-mechanical polishing process for a change in the amplitude or frequency, detecting a change in the amplitude or frequency of the signal component, and altering the chemical-mechanical polishing process in response to the detected change. The invention also provides an apparatus for chemically-mechanically polishing a substrate comprising a chemical-mechanical polishing tool that generates a real-time data signal, wherein the realtime data signal pertains to a frictional force, torque, or motor current, a sensor that transmits the real-time data signal, a data collection unit that receives the real-time data signal, and a signal analysis unit that (i) performs an algorithm on the real-time data signal, (ii) detects a change in the amplitude or frequency of a signal component, and (iii) responds to the detected change to cause an alteration of the chemical-mechanical polishing process.